ALGOL (Beta Persei). Surely one of the most remarkable stars of
the sky and appropriately one of the most famed, Algol is the
second magnitude Beta star of Perseus, the
great mythological hero who rescued Andromeda from Cetus the Sea
Monster. The Arabic name, "al Ghul" (related to our word "ghoul"),
means "the demon," from a longer phrase that refers to the demon's
head. In Greek mythology, Algol represents the Medusa's head with
which Perseus turned Cetus to stone, the star considered an
"unlucky" one for centuries. To the eye, this class B (B8) star
appears rather normal, its slightly bluish white light radiating
from a surface with a temperature of 12,500 K. Like the Sun, it is a main sequence dwarf star fusing
hydrogen in its core, though it is 3.5 times more massive. From
its distance of 90 light years (give or take 4; second Hipparcos
reduction), we calculate a visual luminosity
about 95 times that of the Sun, raised to 170 times if we factor
in the invisible ultraviolet light radiated by the hot surface.
Steady observation, however, reveals a surprise. As regular as
clockwork, every 2.867... days, the brightness of the star plummets
from mid second magnitude (2.1) to the dim end of third (3.4, just
30 percent of normal), the whole event (including recovery) taking
only a few hours.

The brightest star on the left-hand side of each picture is Mirfak, Alpha Persei, while the brightest on
the right-hand side is Algol, Beta Per. In the reference picture at left,
Algol is in its uneclipsed state and nearly as bright at Mirfak.
In picture at right, taken the night of October 9, 2004,
Algol is in mid-eclipse, and notably dimmed.
Compare with other stars in the field as well. The star
immediately to the right of Algol, Rho
Persei is also variable. See full
full resolution. Copyright J. B. Kaler.

Though the variation was discovered in 1667, it was probably known
long before that and is probably the reason for the star's bad
reputation. The cause of the sudden drop is a stellar eclipse.
Algol is a close double star whose components orbit each other
every 2.867... days. The companion to the visually observed star
is a much dimmer yellow-orange class K giant star with a
temperature of 4500 Kelvin and a luminosity 4.5 solar, just 2.5
percent that of the class B star. (The uncertain class of the
faint star ranges from G5 to K2, from subgiant to giant. For
simplicity, let's call it the "K giant.") The B star, at 2.9 solar
radii, is smaller than the K giant (3.5 solar). Each orbit, when
the dimmer, larger K star passes in front of the brighter B star,
we see a deep eclipse. The eclipse is only partial, some of the
light of the principal component still shining brightly through.
Between the deep "primary" eclipses is a smaller dip when the
bright star passes partially in front of the dim one.

The eclipses of Algol were first accurately recorded
photoelectrically around 1910 with the historic 12-inch refractor
at the University of Illinois Observatory. The magnitude scale
does not reflect actual visual magnitudes. The primary eclipse at
left occurs when the bright class B star partially hides behind the
cool K star. The brightness of the system then changes slightly
out of eclipse because of the tidal distortion of the stars and the
reflection of the B star's light from the K star. Thirty hours
after the primary eclipse, the smaller B star passes in front of
the K star and cuts out a portion of its light to create the
secondary eclipse at center. Light curve by Joel Stebbins in the
Astrophysical Journal, vol. 32, p. 185, 1910.

Algol is famed first as a prototype of the class of eclipsing double stars, of
which thousands are known. They are among the most important kinds
of stars, as they provide us with information on stellar masses and
dimensions. But Algol is equally famed for the "Algol paradox."
The higher the mass of a star, the shorter its lifetime, as its
fuel is used so much faster. The companion to Algol is the dying
giant star. Yet carrying but 0.81 solar masses, it is the LESS
massive of the two (the B star weighing in at 3.7 solar). The only
explanation is that the dim companion has lost a great deal of
mass. The two stars are so close together, separated by only five
percent the distance between the Earth and the Sun, that the
brighter smaller star produces tides in the larger one. Matter
then flows in from the large one (at a rate of around two hundred-
millionths of a solar mass per year) to the small bright one, the
effect directly observed through the stellar spectrum as the K
giant is being stripped nearly to its core. A third member of the
system, Algol C, a class A or F star of 1.8 solar masses, orbits
about 3 Astronomical Units away with a period around the inner pair
of 1.86 years. The system is a source of X-rays, though whether
they come from a corona around one of the stars or from the flow of
matter hitting the B star is uncertain. Algol is no demon at all,
but a true friend, teaching us how stars interact and die, the
effects of which you can see from your own backyard with no
telescope at all. (Algol is featured in Jim Kaler's The Hundred
Greatest Stars.)